This invention relates to an improvement in precision length measurement instruments of the type which include a scale, a reading unit moveable along the scale, and a sensor, coupled to the reading unit, for making contact with a surface to be measured.
A large number of references are concerned with length measurement instruments and their errors, specifically with the reduction of these errors. Several specific problems of this area of technology are treated in the descriptive introduction to West German DE-OS No. 16 23 337. Thus, it is desirable that measurement instruments retain the comparator principle, because in an instrument which employs the comparative Abbe principle errors of the first instance are avoided. In some measurement instruments, however, this principle cannot be employed for various technical reasons and thus other arrangements have been developed which employ other measures (such as the Eppenstein principle and the object of the application of DE-OS No. 16 23 337) to eliminate as much as possible the so-called comparison errors.
Furthermore, measurement instruments are known which include spring mounted measurement sensors. The deviation of the measurement sensor from its zero position determines the actual measurement, and the measurement value is given a figure that corresponds to the measurement sensor deviation. Such an instrument is the object of West German DE-PS No. 23 56 030 and is described in the descriptive introduction of that patent.
Such instruments, however, often require relatively complex structures which are not easily operated. Therefore, capital expenditures for this type of instrument tend to be relatively high and such instruments generally require specially qualified personnel to operate them.
For example, in height measurement instruments the Abbe principle generally cannot be used. Because geometrical considerations often dictate a comparator distance of 50 to 100 mm, one must, for example, maintain a guide precision of .+-.2 arc seconds in order to have a measurement precision of .+-.1 micron. In order to keep manufacturing costs at a practical level the use of light metal profiles seems indicated but their straightness, however, remains substantially under the above mentioned value.
The requirement for high measurement precision also means that the measurement carriage must be moved with little friction. In this way measurement forces can be maintained constant within narrow limits in order to avoid deformations of the measurement sensor caused by variations in measuring forces. Roller bearings, generally used for low friction guides, are often not sufficiently precise as a result of the inevitable construction tolerances and the gradual deposition of foreign material on the guide surfaces.